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Inhaltsverzeichnis 1 Santa’s helper is in a bad mood 3 1.1 Exercise ................................ 3 1.2 Solution ................................ 6 2 Disguised Thieves 7 2.1 Exercise ................................ 7 2.2 Solution ................................ 10 3 Snowman made of glass 12 3.1 Exercise ................................ 12 3.2 Solution ................................ 15 4 Transportation costs 20 4.1 Exercise ................................ 20 4.2 Solution ................................ 22 5 The Big Reindeer Race 24 5.1 Exercise ................................ 24 5.2 Solution ................................ 27 6 Explosion 29 6.1 Exercise ................................ 29 6.2 Solution ................................ 31 7 Cookies 33 7.1 Exercise ................................ 33 7.2 Solution ................................ 35 8 Distributing presents in Hawaii 36 8.1 Exercise ................................ 36 8.2 Solution ................................ 39 9 Brain teaser 44 9.1 Exercise ................................ 44 9.2 Solution ................................ 46 10 The Christmas cookie dice game 47 10.1 Problem ................................ 47 10.2 Solution ................................ 49 11 Card game 52 11.1 Exercise ................................ 52 11.2 Solution ................................ 54 12 The radio competition 55 12.1 Exercise ................................ 55 12.2 Solution ................................ 57 2 13 Test for a job interview 59 13.1 Exercise ................................ 59 13.2 Solution ................................ 61 14 Deep snow 62 14.1 Exercise ................................ 62 14.2 Solution ................................ 64 15 Crossnumbers 66 15.1 Exercise ................................ 66 15.2 Solution ................................ 69 16 A secure safe 71 16.1 Exercise ................................ 71 16.2 Solution ................................ 74 17 Hats 76 17.1 Exercise ................................ 76 17.2 Solution ................................ 79 18 The dance of the elves 81 18.1 Exercise ................................ 81 18.2 Solution ................................ 83 19 Mondrian 87 19.1 Exercise ................................ 87 19.2 Solution ................................ 89 20 Mixed Doubles 91 20.1 Exercise ................................ 91 20.2 Solution ................................ 94 21 Raiders of the lost Santa 97 21.1 Exercise ................................ 97 21.2 Solution ................................100 22 Temperature 102 22.1 Exercise ................................102 22.2 Solution ................................104 23 A Christmas Crime Story 105 23.1 Exercise ................................105 23.2 Solution ................................108 24 Mosquito 110 24.1 Exercise ................................110 24.2 Solution ................................112 3 1 Santa’s helper is in a bad mood Author: Christian Hercher 1.1 Exercise Santa’s helper is in a bad mood. “Same procedure as last year? Same procedure as every year...” *grrr* “I’m always taking care of the naughty kids, scaring them and the old man gets all the recognition for the good kids’ gifts.” This time, everything will be different. And off he goes to Santa Claus and cries on his shoulder. “What are you suggesting?”, asks Santa. “It’s obvious! I want to switch this year!” Of course, Father Christmas doesn’t give up his privilege so easily! “Let us play for it. The winner gets to distribute the gifts and the loser visits the naughty boys and girls.” “Ok, and what are we playing?” answers Santa’s helper, interested. The old man explains the game he has in mind. “In my gift bag, there are 23 red gifts and 23 blue gifts. Other than by the color, you can’t tell them apart.” Without looking inside, you pull two gifts out of the gift bag. If they possessed different colors, you must put the red one back in. On the other hand, if they have the same color, a blue gift is added to the gift bag. (I have enough extra blue gifts that are not in the gift bag yet, so you can use them for the game.) 4 This procedure is repeated until there are no more gifts in the gift bag. The color of the last gift that you have to put back in the bag after the last pull according to our rules decides the winner of the game. If it is blue, you win. If it is red, I am declared the winner.” The little helper ponders a little. “Why do you get to play red?” - “Because I am Santa Claus. Besides, I am the one with the red suit that all the kids love! So red is my color.” “Okay, alright! ...” he responds after giving up. “I feel like I’m being scammed, but I’ll give it a try. Let’s start!” With what probability (rounded at multiples of 10 %) will Santa’s helper win the game and become the kids’ hero this year? 5 Possible Answers: 1.0% 2. 10 % 3. 20 % 4. 30 % 5. 40 % 6. 50 % 7. 60 % 8. 80 % 9. 90 % 10. 100 % 6 1.2 Solution Answer 1: The probability for Servant Rupert to win the game is ex- actly zero. After every turn, two gifts are pulled out of the bag and one gift is added back in. The number of gifts in the bag thus goes down by one per turn, which means the game ends when the two last gifts are pulled out of the bag, in which case the color of the gift to put back in decides the winner. When two gifts of different colors are pulled out, the number of red gifts in the bag doesn’t change. This is also the case when two blue gifts are pulled out since a blue gift is added back in. The number of red gifts in the bag can only change when two red gifts are taken out, but since a blue gift goes back in, the number of red gifts in the bag diminishes by exactly two. At the beginning, there are 23 red gifts in the bag, which is an odd number. Since every step of the game removes either zero or two red gifts from the bag, the number of red gifts in the bag remains odd at each step since it was odd in the previous step. In particular, there is always at least one red gift in the bag. This has to be the color of the last gift that has to be put back in, so Santa Claus wins for sure. The winning probability of Santa’s little helper is therefore exactly 0 %. Note: the strategy used to solve this problem is also called “Invariance Principle”. One finds a property which remains invariant through every step of the game and gives us the solution at the last step. In this case, the invariant property is the oddness of the number of red gifts in the bag, no matter which gifts are pulled out of it. 7 2 Disguised Thieves Author: Thomas Lütteke 2.1 Exercise Santa is furious. Since he doesn’t enjoy a good wine only when Christmas is ap- proaching, he stores a few bottles in his wine cellar. However, after the Christmas elves’ costumed Halloween party, some of his best bottles went missing. Consi- dering this is not the first time this has happened, he decided to install a video surveillance camera. To his dismay, he could not recognize the thieves on the tape because they were still costumed: one of them is dressed as an Easter bunny, the other one - and that particularly outrages him - dressed up in a Santa Claus costume! Such an enormous one on top of that, as if he had a gigantic belly. Of course, now he wants to find out who was hiding behind those costumes. Actually, he brought it down to five suspects, namely Alex, Benni, Chris, Dieter and Emil. Altogether, they were costumed as Santa, the Easter bunny, a monk, Yoda and Snow White. All five of them sat next to each other at the bar almost all night in the same order and each one of them kept drinking the same thing: one drank vodka, one took milk, one took beer, the other one had a Coca-Cola and the last one had lemonade. As suspects, all five of them made use of their right to remain silent. The other elves at the party were also questioned, but they were pretty drunk and therefore did not have much information to offer. All that Santa got out of them was the following statements: • The tall Alex only fits into the Santa or monk costumes. • Dieter always drinks Coca-Cola or vodka at parties. • Either Snow White or the monk was sitting at the right end of the counter. • The elf at the second left position drank beer or lemonade. 8 • The two elves at the right end of the counter did not drink milk. • The Easter bunny was sitting either at the left end of the bar or exactly in the middle. • Benni was sitting on Santa’s right and on the left of the elf who drank Coca-Cola. • Emil was sitting to the right of the elf who drank Coca-Cola and to Snow White’s left. • Chris did not sit in the middle seat. • Emil is too tall to fit into the Yoda costume and he likes neither lemonade nor vodka. • The monk did not drink milk. Is it possible with this information to figure out the thieves’ identities, or at least reduce the number of suspects by definitely excluding individual Christmas elves? 9 Possible Answers: 1. The thieves are Alex and Chris. 2. The thieves are Benni and Dieter. 3. The thieves are Chris and Emil. 4. The thieves are Dieter and Alex. 5. The thieves are Emil and Benni. 6. Santa Claus can only identify one thief with certainty and only one more elf can definitely be excluded. 7. Santa Claus can only identify one thief with certainty and two further elves can definitely be excluded. 8. Santa Claus cannot identify any of the thieves. All he can do is definitely exclude one of the elves. 9. The information given is insufficient to convict or exonerate any of the elves. 10. The information given is contradictory, i.e. at least one piece of information must be incorrect.

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